Census Shows Where Milkweed
Technician Keith Kohler (left) and weed scientist
Bob Hartzler measure vegetative characteristics of common milkweed
found along an Iowa roadside.
|Conservationists and others concerned
about the fate of the monarch butterfly may be heartened by a recent survey of
milkweed distribution in the major U.S. corn-growing area.
In May 1999, new questions arose about Bacillus thuringiensis
(Bt), a soil bacterium used for 35 years as an effective alternative to
chemical insecticides for controlling moth pests. Its useeither as a
spray or through genetic insertion into cropshas grown over the years
because it reputedly doesn't harm humans, animals, beneficial insects, or other
But a note published in Nature questioned that: A small, preliminary
laboratory experiment suggested that Bt corn pollen has a negative
effect on monarch butterflies because of its drift onto nearby milkweed plants,
a primary food source for the butterfly.
Through an intensive survey of the Iowa rural
landscape, ARS researchers found common milkweed growing in 71
percent of the roadside areas.
|In laboratory feeding experiments,
monarch larvae fed milkweed leaves artificially coated with pollen from
Bt corn ate less, grew more slowly, and suffered a higher death rate
than larvae that consumed milkweed free of the pollen.
Bt transgenic plants have been used since 1996 because farmers get
excellent crop yields without spraying pesticides. The note in Nature
raised some concern about the crop's possibly harming nontarget species in
areas where Bt corn is grown extensively.
"Common milkweed, Asclepias syriaca, is a plant native to the
northeastern and north central United States and adjacent areas of Canada. It's
an important component of the life cycle of the monarch butterfly, Danaus
plexippus," says Douglas D. Buhler. Formerly an
ARS agronomist at the National Soil
Tilth Laboratory in Ames, Iowa, he now chairs the Department of Crop and Soil
Sciences at Michigan State University-East Lansing.
Monarch butterfly larva feeds on leaves of common
| "Monarchs lay their eggs
exclusively on plants in the milkweed family, Asclepiadaceae, usually on the
underside of a leaf," says Buhler. "There are over 100 milkweed
species in North America, but common milkweed is the most prevalent and the
main host for monarchs in the Midwest.
"Common milkweed and other closely related plant species are the sole food
source of monarch larvae, or caterpillars," he says. "They feed on
milkweed foliage, flower buds, and milky juice. The importance of common
milkweed populations in the central United States to the monarchcoupled
with concerns about toxicity to its larvae of pollen from one variety of
Bt corn deposited on milkweed leaveshas generated considerable
interest in knowing how milkweed is distributed in croplands and adjacent
Buhler adds, "We also need to know the larvae's potential for exposure to
the pollen in the real world, so we must determine where milkweed grows in
relation to corn fields."
A Different Kind of Census
To determine the relative distribution of common milkweed in different habitats
in Iowa, Buhler, working with ARS technician Keith A. Kohler and Iowa State
University-Ames weed scientist Robert G. Hartzler, conducted the first
intensive survey of the state's farmlands and adjacent areas. They wanted to
better understand the role of this native plant's ecology in the life cycle of
"In June and July 1999, using a random, representative sample of Iowa
farmlands, we selected 40 areas that were each 38.6 square miles, using a
coordinate reference system," says Kohler. "We were interested in
knowing the population dynamics and geographic distribution of common
Within these sample units, the team arbitrarily selected 10 areas, each 55
yards by 110 yards. The areas included all possible habitats where the weed
commonly growscorn and soybean fields, pastures, roadsides, and land
maintained in the Conservation Reserve Program (CRP) of the USDA Natural
Resources Conservation Service.
The researchers found common milkweed growing along 71 percent of the roadsides
and in about half of the corn and soybean fields sampled. Pastures had the
lowest occurrence of milkweed28 percent. Corn and soybean fields had 85
percent fewer patches of common milkweed than roadsides.
"While common milkweed was often found in corn and soybean fields, the
average frequency and patch sizes were much greater along roadsides and in
other undisturbed areas," says Buhler. "Roadsides had 22 patches per
acre, whereas corn and soybeans averaged 3."
According to Buhler, "CRP fields had relatively few patchesabout
four per acre. However, these patches were larger than those in other settings.
This resulted in CRP land having the highest percentage2 percentof
area infested with common milkweed."
He says, "Land in cornfields, soybean fields, or pastures had the lowest
average area infestedless than 0.03 percentbut these land uses
cover 78 percent of the Iowa land mass."
These results are of great interest to those studying the proportion of
milkweed population that may be affected by changing herbicide-use patterns in
crop fieldsand the potential impact of Bt pollen on monarch
populations. The study will be of great interest to scientists studying the
fate of Bt corn.
This research also provides vital information about the distribution of common
milkweed in Iowathe major U.S. corn-producing stateand in areas
with similar climate and land-use patterns. But, says Buhler, "Additional
information is needed before an accurate assessment can be made of the impact
of genetically modified crops on monarch butterflies."By
Becker, Agricultural Research Service Information Staff.
This research is part of Soil Resource Management, an ARS National Program
(# 202) described on the World Wide Web at
Douglas D. Buhler is with the
Department of Crop and Soil Sciences at
Michigan State University, East Lansing, MI 48824; phone (517) 355-0217,
fax (517) 353-5174.
Keith A. Kohler is with the
National Soil Tilth Laboratory, 2150 Pammel
Dr., Ames, IA 50011-3120; phone (515) 294-8240, fax (515) 294-8125.
"Census Shows Where Milkweed Grows" was
published in the December 2000
issue of Agricultural Research magazine.